Snowmobiles are popular land vehicles used as transportation vehicles or as recreational vehicles in cold and snowy conditions. In general, a snowmobile has a central frame or chassis on or around which the various components of the snowmobile are assembled. Typical snowmobiles include skis for steering, a seat, handlebars, and an endless track for propulsion mounted to a central chassis. An engine cradle or bulkhead is defined by a plurality of front structural members of the chassis. The engine drives a ground-engaging endless track disposed in a longitudinally extending drive tunnel formed within the chassis. The skis serve to facilitate steering as well as to provide flotation of the front of the snowmobile over the snow in which it is operated. The skis are mounted at the front body portion of the chassis. A handlebar assembly, positioned forward of the seat, is operatively linked to the skis for steering the snowmobile. The skis may be pivoted laterally to steer the snowmobile, for example, by turning the handlebars.
Snowmobiles typically have a front steering ski and an endless drive track carried by a subframe rearwardly of the ski, with the subframe being resiliently suspended from the main frame of the snowmobile. The track suspension system of a snowmobile is important because it determines, at least to some extent, the vehicle's shock-absorbing capability, stability, handling, traction, and power requirements.
Snowmobile track suspension systems are of three general types. One type is a bogie wheel suspension, in which sets of bogie wheels are carried by axles mounted on a resiliently suspended subframe to engage the lower run of the drive track so as to tend to maintain the track in driving engagement with the snow. The bogie wheel suspension system involves mostly rolling friction, with very little sliding friction between the bogie wheels and the track, thereby requiring little, if any, lubrication. Typical bogie wheel suspensions are shown, for example, in U.S. Pat. Nos. 3,773,126 and 3,658,145.
A second type of track suspension system is the slide rail suspension, in which the lower run of the drive track travels in sliding engagement with a pair of longitudinally extending rails. Snow acts as a lubricant between the track and rails to reduce friction. The slide rail suspension tends to hold the track flatter against the snow in rough or variable terrain, which can be important when speed or power is required, such as in racing. Typical slide rail suspensions are shown, for example, in U.S. Pat. Nos. 3,690,394, 3,738,714, 3,974,890, and 3,758,170.
A third type of track suspension system is a combination of the slide rail and bogie wheel suspensions in a single subframe assembly. Bogie wheels act to hold at least portions of the lower run of the track away from the slide rail to reduce sliding friction.
In most known track suspension systems of either the bogie wheel, slide rail, or combination type, the bogie wheels and slide rails are carried by a subframe which is resiliently suspended from the snowmobile's main frame so as to provide shock-absorbing capability and thereby isolate the main frame from terrain-induced shock. The track on such subframes is driven by a at least one drive sprocket mounted on a drive axle. The drive axle is carried by the main frame of the snowmobile, as shown in the aforementioned patents. While the track drive axle and sprockets are located at one end of the track, there is usually an idler axle carrying a set of idler wheels or guide wheels at the opposite end of the track. The idler axle may be carried by either the subframe or the main frame. Additionally, carrier wheels may be positioned in an upper portion of the subframe to further guide the track in its path around the subframe. Therefore, the resilient means which suspends the subframe from the main frame does not isolate the entire length of the track from the main frame, resulting in some shock and vibration being transmitted through the fore and aft portions of the track and its axles to the main frame.
The drive track assembly comprises an endless track (a continuous loop of material) with an outer surface for engagement with the ground and a plurality of track rods. The track rods are spaced longitudinally along the endless track such that movement of the endless track as it provides vehicle propulsion causes the track rods to repeatedly impact the bogie wheels as they move relative to each other. This may occur on track suspension systems of the bogie wheel or combination bogie wheel/slide rail type. Each instance of a bogie wheel traveling over a track rod causes a jarring force or impact to the wheel that creates unwanted noise and vibration, which can be transmitted to the main frame (and hence, to the operators and passengers), as described above. Additionally, each instance of a track rod moving over a carrier wheel may cause similar noise and vibration.
The effect of the noise and vibration from track rods impacting bogie wheels and/or carrier wheels may be compounded in many snowmobile suspensions, due to the positioning of such wheels in symmetric pairs along the longitudinal axis. Thus, in typical snowmobile suspensions, there is an additive effect of the noise caused by having two or more bogie wheels and/or carrier wheels impacting track rods at the same instant in time. The simultaneous occurrence of wheels impacting track rods creates a repetitive “jarring” noise component that may be unpleasant to snowmobile operators and passengers, and which may reduce drive-train efficiency (and hence performance) of the snowmobile.